Add support for `squash_merge_commit_title` and `squash_merge_commit_message`

[gr2m]

What’s missing?

3 new properties have been introduced to further customize the squash merge commit message:

1. squash_merge_commit_title
2. squash_merge_commit_message

Why?

Because currently the commit message defaults to the titles of all commits.

Alternatives you tried

n/a